Parabolic reflectors (or paraboloids) and mirrors are a spherical cap with a polished, well-reflecting surface. The paraboloid has the property that an on-axis parallel beam of radiation will be reflected by the surface and concentrated at its focus (or conversely, a point source located at the focus will produce a parallel beam on reflection).
Parabolic mirrors form two types of images of objects: real and virtual images. If the object is placed on the axis of revolution and further from the surface of the mirror than the focal point, the image formed is the real image. If the object is between the mirror and the focal point a virtual image is formed. If the object is placed at the focal point no image is formed.
Mirrors constituted by a glass sheet having a reflective coating of, for example, silver or black are of course very well known. Particularly in the case of large mirrors, they suffer from the disadvantage that they must often be made unduly heavy in order to reduce the risk of their breaking, and the weight and thickness of glass required can make such mirrors unsuitable for use in some circumstances.
It has been recognized that it has been difficult to mechanically deform a rigid mirror into a precise concave shape and maintain it in that concave shape without it having defects which might vary the focal point of the mirror. This problem is particularly prevalent in constructing paraboloid shaped mirrors. Consequently, in constructing concave mirrors, particularly of large focal length, the most frequently suggested construction techniques have been to either mold a reflective facing layer and a holding layer into the desired shape, or to apply a reflective layer as a coating on a surface which has already been formed in the desired configuration.
Accordingly, it is desirable to develop a method of manufacturing a parabolic mirror with a large focal length without it having defects which might vary the focal point of the mirror.